Mineral-oil base oil, lubricating oil composition, internal combustion engine, and lubricating method for internal combustion engine

ABSTRACT

Disclosed herein is a mineral base oil satisfying the following requirements (I) to (V): (I): a kinematic viscosity at 100° C. is 2 mm2/s or more and less than 7 mm2/s; (II): a viscosity index is 100 or more; (III): a temperature gradient Δ|η*| of complex viscosity between two points of −10° C. and −25° C. is 60 Pa·s/° C. or less as measured with a rotary rheometer under conditions at an angular velocity of 6.3 rad/s and a strain amount of 0.1 to 100%; (IV): a content ratio [R1/R2] of a monocyclo paraffin component (R1) to a dicyclo to hexacyclo paraffin component (R2) is 0.70 or less by a volume ratio as measured in accordance with ASTM D2786; and (V): a % CA is less than 1.0.

TECHNICAL FIELD

The present invention relates to a mineral base oil, a lubricating oil composition using the mineral base oil, an internal combustion engine using the lubricating oil composition, and a method for lubricating the internal combustion engine.

BACKGROUND ART

In recent years, hybrid vehicles and vehicles equipped with a start-stop mechanism have increased. In these vehicles, the temperature of the engine oil cannot be easily increased. The engine oils used for these vehicles are thus particularly required to further improve low-temperature viscosity characteristics such that fuel consumption and engine start-up performance at a lower temperature are improved.

In addition to such low-temperature viscosity characteristics, the engine oils are also required to have other desirable properties, including a viscosity-temperature characteristic, and low evaporativity.

A lubricant base oil used for an engine oil has been actively developed in order to provide an engine oil with these improved characteristics in a well-balanced manner. For example, PTLs 1 to 4 disclose lubricant base oils in which specific physical property values are adjusted within a predetermined range.

CITATION LIST Patent Literature

PTL 1: JP 2008-274237 A

PTL 2: JP 2012-153906 A

PTL 3: JP 2007-016172 A

PTL 4: JP 2006-241436 A

SUMMARY OF INVENTION Technical Problem

Typically, in order to obtain an engine oil with improved low-temperature viscosity characteristics, it has been generally conducted to improve the low-temperature viscosity characteristics of the engine oil by blending a lubricant base oil with a pour-point depressant or a viscosity index improver, which is a polymer component.

However, the presence of such a polymer component to be mixed as a pour-point depressant or a viscosity index improver becomes a factor that lowers the high-temperature piston detergency of the engine oil.

Engine oils using the lubricant base oils described in PTLs 1 to 4 involve problems in high-temperature piston detergency and also have room for a more improvement in the low-temperature viscosity characteristics.

Accordingly, there is a need for an engine oil having improved low-temperature viscosity characteristics and high-temperature piston detergency in a good balance.

An object of the present invention is to provide a mineral base oil that can be used as an engine oil having desirable low-temperature viscosity characteristics, including low-temperature fuel consumption and low-temperature engine start-up performance, and also having excellent high-temperature piston detergency, a lubricating oil composition using the mineral base oil, an internal combustion engine using the lubricating oil composition, and a method for lubricating an internal combustion engine with the lubricating oil composition.

Solution to Problem

The present inventors have found that the foregoing problems can be solved with a mineral base oil prepared so as to satisfy specific requirements.

The present invention has been accomplished on the basis of this finding.

That is, the present invention provides the following [1] to [4].

[1] A mineral base oil satisfying the following requirements (I) to (V).

-   -   Requirement (I): a kinematic viscosity at 100° C. is 2 mm²/s or         more and less than 7 mm²/s;     -   Requirement (II): a viscosity index is 100 or more;     -   Requirement (III): a temperature gradient Δ|η*| of complex         viscosity between two points of −10° C. and −25° C. is 60         Pa·s/° C. or less as measured with rotary rheometer under         conditions at an angular velocity of 6.3 rad/s and a strain         amount of 0.1 to 100%;     -   Requirement (IV): a content ratio [R1/R2] of a monocyclo         paraffin component (R1) to a dicyclo to hexacyclo paraffin         component (R2) is 0.70 or less by a volume ratio as measured in         accordance with ASTM D2786; and     -   Requirement (V): a % C_(A) is less than 1.0.         [2] A lubricating oil composition containing the mineral base         oil as set forth in above [1].         [3] An internal combustion engine including a sliding mechanism         equipped with a piston ring and a liner; and the lubricating oil         composition as set forth in above [2].         [4] A method of lubricating an internal combustion engine having         a sliding mechanism equipped with a piston ring and a liner, the         method including lubricating the piston ring and the liner are         lubricated with the lubricating oil composition as set forth in         above [2].

Advantageous Effects of Invention

A lubricating oil composition having desirable low-temperature viscosity characteristics, including low-temperature fuel consumption and low-temperature engine start-up performance, and also having excellent high-temperature piston detergency can be easily prepared by using the mineral base oil according to the present invention.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph illustrating the relationship between temperature and complex viscosity η* with respect to the mineral base oil (2) of Example 2, the mineral base oil (a) of Comparative Example 1, and the mineral base oil (b) of Comparative Example 2.

FIG. 2 is a schematic view illustrating an outline of a configuration of a sliding mechanism equipped with a piston ring and a liner.

DESCRIPTION OF EMBODIMENTS

In this specification, the values of kinematic viscosity and viscosity index at predetermined temperatures are values measured in accordance with JIS K2283:2000.

In this specification, the value of the complex viscosity η* at a predetermined temperature is a value measured with a rotary rheometer under conditions at an angular velocity of 6.3 rad/s and a strain amount of 0.1 to 100%, and more specifically, means a value measured according to the method described in the section of Examples. The aforementioned “strain amount” is a measurement condition parameter that is appropriately set within a range of from 0.1 to 100% according to the measurement temperature.

In this specification, the values of the weight average molecular weight (Mw) and the number-average molecular weight (Mn) of the respective component are each a value expressed in terms of standard polystyrene as measured by the gel permeation chromatography (GPC), specifically, a value measured according to the method described in the section of Examples.

In the present specification, the CCS viscosity (low-temperature viscosity) at −35° C. is a value measured in accordance with JIS K 2010: 1993 (ASTM D 2602).

In the present specification, the naphthene content (% C_(N)) and the aromatic content (% C_(A)) of a mineral base oil are ratios (percentage) of the naphthene content and the aromatic content, respectively, as measured by ASTM D-3238 ring analysis (n-d-M method).

[Mineral Base Oil]

Examples of a feedstock for the mineral base oil of the present invention include: an atmospheric bottom oil obtained by subjecting a crude oil, such as a paraffin-based crude oil, an intermediate-based crude oil, and a naphthene-based crude oil to atmospheric distillation; distillate and wax obtained by subjecting the atmospheric bottom oil to distillation under reduced pressure; and a GTL wax obtained by Fischer Tropsch synthesis of natural gas.

Moreover, the mineral base oil of the present invention may be obtained by subjecting these feedstocks to one or more of purification processes, such as solvent deasphalting, solvent extraction, hydrogenation process, solvent dewaxing, catalytic dewaxing, and hydroisomerization dewaxing.

These mineral oils may be used either alone or in combination of two or more thereof.

The mineral base oil of the present invention satisfies the following requirements (I) to (V).

-   -   Requirement (I): a kinematic viscosity at 100° C. is 2 mm²/s or         more and less than 7 mm²/s.     -   Requirement (II): a viscosity index is 100 or more.     -   Requirement (III): a temperature gradient Δ|η*| of complex         viscosity between two points of −10° C. and −25° C. is 60         Pa·s/° C. or less as measured with a rotary rheometer under         conditions at an angular velocity of 6.3 rad/s and a strain         amount of 0.1 to 100%.     -   Requirement (IV): a content ratio [R1/R2] of a monocyclo         paraffin component (R1) to a dicyclo to hexacyclo paraffin         component (R2) is 0.70 or less by a volume ratio as measured in         accordance with ASTM D2786.     -   Requirement (V): a % C_(A) is less than 1.0.

In addition, it is preferred that the mineral base oil of one embodiment of the present invention further satisfies the following requirement (VI).

-   -   Requirement (VI): complex viscosity η* at −35° C. is 60,000 Pa·s         or less as measured with a rotary rheometer under conditions at         an angular velocity of 6.3 rad/s and a strain amount of 0.1%.

In the case where the mineral base oil of one embodiment of the present invention is a mixed oil of two or more mineral oils, it is enough that the mixed oil satisfies the aforementioned requirements.

The requirements (I) to (VI) are hereunder described.

<Requirement (I)>

The requirement (I) is one prescribing the balance between the evaporation loss and the fuel economy improving effect of the mineral base oil.

Namely, when the kinematic viscosity at 100° C. of the mineral base oil of the present invention is less than 2 mm²/s, the evaporation loss increases, and hence, such is not preferred. On the other hand, when the kinematic viscosity at 100° C. is 7 mm²/s or more, the power loss to be caused due to viscosity resistance increases, and hence, such is problematic in terms of a fuel economy improving effect.

From the viewpoint of reducing the evaporation loss of the mineral base oil, the kinematic viscosity at 100° C. of the mineral base oil of one embodiment of the present invention is preferably 2.1 mm²/s or more, more preferably 2.2 mm²/s or more, and still more preferably 2.5 mm²/s or more, and from the viewpoint of improving the fuel economy improving effect of the mineral base oil, it is preferably 6 mm²/s or less, more preferably 5.5 mm²/s or less, still more preferably 5 mm²/s or less, and yet still more preferably 4.7 mm²/s or less.

<Requirement (II)>

The requirement (II) is a prescription for producing a mineral base oil with a desirable viscosity-temperature characteristic and desirable fuel consumption.

Namely, when the viscosity index of the mineral base oil of the present invention is less than 100, the viscosity-temperature characteristic and fuel consumption notably decrease, and a lubricating oil composition using the mineral base oil becomes problematic in terms of a fuel consumption performance.

From the foregoing viewpoint, the viscosity index of the mineral base oil of one embodiment of the present invention is preferably 105 or more, and more preferably 110 or more.

In addition, the mineral base oil of the present invention satisfies the requirement (III) as described later, and therefore, even when its viscosity index is not relatively high, a lubricating oil composition having desirable low-temperature viscosity characteristics, including low-temperature fuel consumption and low-temperature engine start-up performance can be provided.

Accordingly, the viscosity index of the mineral base oil of one embodiment of the present invention is preferably 145 or less, more preferably 140 or less, still more preferably 135 or less, and yet still more preferably less than 133.

<Requirement (III)>

As prescribed by the requirement (III), the mineral base oil of the present invention requires that the temperature gradient Δ|η*| of complex viscosity between two temperature points −10° C. and −25° C. (hereinafter also referred to simply as “temperature gradient Δ|η*| of complex viscosity”, unless otherwise specified) is 60 Pa·s/° C. or less as measured with a rotary rheometer under conditions at an angular velocity of 6.3 rad/s and a strain amount of 0.1 to 100%.

The value of the aforementioned “strain amount” in the requirement (III) is appropriately set within a range of from 0.1 to 100% according to the temperature.

The aforementioned “temperature gradient Δ|η*| of complex viscosity” is a value indicative of an amount of change (absolute value of a slope) of complex viscosity per unit between two temperature points −10° C. and −25° C. as observed when the value of the complex viscosity η* at −10° C. and the value of the complex viscosity η* at −25° C. as measured either independently at these temperatures or while continuously varying the temperature from −10° C. to −25° C. or from −25° C. to −10° C. are placed on a temperature-complex viscosity coordinate plane. More specifically, the temperature gradient Δ|η*| of complex viscosity means a value calculated from the following calculation formula (f1).

Temperature gradient Δ|η*| of complex viscosity=|([complex viscosity η* at −25° C.]−[complex viscosity η* at −10° C.])/(−25−(−10))≡  Calculation formula (f1):

The present inventors have found that by specifically associating the complex viscosity of the mineral base oil with the temperature, effects that low-temperature viscosity characteristics, including low-temperature fuel consumption and low-temperature engine start-up performance, and piston detergency are excellent are obtained; and that the relationship between complex viscosity and temperature is greatly influenced by the components, the composition, the state, the manufacturing conditions, and so on of the mineral base oil.

FIG. 1 is a graph representing the relationship between temperature and complex viscosity η* with respect to the mineral base oil (2) of Example 2, the mineral base oil (a) of Comparative Example 1, and the mineral base oil (b) of Comparative Example 2, as described later.

The “temperature gradient Δ|η*| of complex viscosity” as referred to herein is the amount of change of complex viscosity over a temperature range of from −25° C. to −10° C., namely the slope of the graph shown in FIG. 1.

In general, as one of evaluation indexes of low-temperature viscosity characteristics, a “pour point” that is a temperature just before the mineral base oil solidifies is used.

The present inventors have found that the temperature at which the complex viscosity rapidly increases is substantially coincident with the “pour point”; and that even in mineral oils having a “pour point” close to each other, as shown in the graph of FIG. 1, the mineral oils differently exhibit increases or decreases of the complex viscosity in a low-temperature environment below the pour point.

On the basis of these findings, the present inventors have envisaged that it might be possible to obtain a mineral base oil with improved low-temperature viscosity characteristics when a specified relationship is considered between the complex viscosity of the mineral base oil and the temperature in a low-temperature environment below the pour point, thereby leading to accomplishment of the present invention.

Other typical evaluation methods of low-temperature viscosity characteristics use values of various viscosities, such as CCS viscosity, BF viscosity, etc. However, these evaluation methods do not necessarily accurately specify the low-temperature viscosity characteristics of a mineral base oil in a low-temperature environment.

Namely, a mineral base oil contains a wax, and the oil forms a gelatinous structure as the wax component precipitates in a low-temperature environment below the pour point. The gelatinous structure easily breaks, and the viscosity changes under a mechanical action. Accordingly, the CCS viscosity used to evaluate the low-temperature viscosity characteristics is thus merely a low-temperature apparent viscosity under predetermined conditions, and does not represent a physical property that sufficiently represents the viscosity characteristics in a low-temperature environment.

In addition, in order to prepare a mineral base oil so as to satisfy the requirement (IV) to be described below, for example, when the mineral base oil is purified by using a feedstock oil containing a bottom oil, in the resulting mineral base oil, a measured value is easily unstable or is affected in some (for example, measurement of the BF viscosity) of the methods of evaluating the low-temperature viscosity characteristics, and in such a case, the low-temperature viscosity characteristics cannot be accurately evaluated.

Then, the present inventors made various extensive and intensive investigations. As a result, it has been found that by focusing on the aforementioned “temperature gradient Δ|η*| of complex viscosity”, a mineral base oil with improved low-temperature viscosity characteristics can be obtained by considering the changes in coefficient of friction following the precipitation of the wax component, while taking into account the precipitation rate of the wax component contained in the mineral base oil, which cannot be grasped with CCS viscosity, BF viscosity, and so on, thereby leading to accomplishment of the present invention.

In accordance with the investigations made by the present inventors, a mineral base oil having the temperature gradient Δ|η*| of complex viscosity exceeding 60 Pa·s/° C. involves a high wax precipitation rate, and is liable to cause an increase of coefficient of friction. As a result, it has been found that a lubricating oil composition using the foregoing mineral base oil has a poor fuel saving performance in a low-temperature environment.

Furthermore, the present inventors have also found that a lubricating oil composition (engine oil) with greatly improved high-temperature piston detergency can be prepared by using a mineral base oil having a small temperature gradient Δ|η*| of complex viscosity.

Namely, it has been noted that a lubricating oil composition using a mineral base oil having a temperature gradient Δ|η*| of complex viscosity of 60 Pa·s/° C. or less can have desirable high-temperature piston detergency, as shown in the section of Examples as described later. In addition, such a lubricating oil composition produces only a few deposits and can have desirable piston detergency even when a polymer component, such as a pour-point depressant, etc., that may cause deposit production, is added together with the mineral base oil having a temperature gradient Δ|η*| of complex viscosity of 60 Pa·s/° C. or less.

From the above viewpoint, in the mineral base oil of one embodiment of the present invention, the temperature gradient Δ|η*| of complex viscosity defined in the requirement (III) is preferably 50 Pa·s/° C. or less, more preferably 20 Pa·s/° C. or less, and even more preferably 15 Pa·s/° C. or less.

In addition, in the mineral base oil of one embodiment of the present invention, in the temperature gradient Δ|η*| of complex viscosity defined in the requirement (III), the lower limit value thereof is not particularly limited, but is preferably 0.001 Pa·s/° C. or more, and more preferably 0.01 Pa·s/° C. or more, and from the viewpoint of obtaining a mineral base oil from which a lubricating oil composition with further improved piston detergency is easily prepared, the lower limit value thereof is more preferably 0.1 Pa·s/° C. or more, even more preferably 1.0 Pa·s/° C. or more, and particularly preferably 1.5 Pa·s/° C. or more.

<Requirement (IV)>

In the mineral base oil of the present invention, as defined in the requirement (IV), a content ratio [R1/R2] of a monocyclo paraffin component (R1) to a dicyclo to hexacyclo paraffin component (R2) needs to be 0.70 or less by a volume ratio as measured in accordance with ASTM D2786.

A cyclic paraffin component is also called a “naphthene component,” and corresponds to a monocyclo paraffin component, such as cyclopentane and cyclohexane, or a product obtained by bonding or condensing two or more rings of these monocyclo paraffin components.

The cyclic paraffin component also includes those in which hydrogen atoms bonded to ring carbon atoms forming a cyclic structure are substituted with various substituents.

Further, the cyclic paraffin component includes an unsaturated alicyclic compound, such as cyclopentene and cyclohexene containing a double bond in the cyclic structure, but does not include an aromatic compound.

In general, it is known that the cyclic paraffin content (naphthene content) contained in the mineral base oil causes a decrease in viscosity index.

Accordingly, since the mineral base oil used in the engine oil is required to have good viscosity characteristics over a wide temperature range, a mineral base oil having a low cyclic paraffin component (naphthene component) (specifically, a base oil having a % C_(N) of less than 15) is regarded as suitable.

According to the studies of the present inventors, it was found that the cause of the decrease in viscosity index was due to the presence of the “dicyclo to hexacyclo paraffin component (R2).”

Meanwhile, it was also found that the presence of the dicyclo to hexacyclo paraffin component (R2) contributes to improvement in detergency.

The present inventors conducted extensive studies on a mineral base oil which suppresses a decrease in viscosity index, has good low-temperature viscosity characteristics, and can further improve detergency.

Moreover, it was found that when a mineral base oil is prepared so as to satisfy the requirement (III), and a mineral base oil prepared so as to have a content ratio [R1/R2] of the monocyclo paraffin component (R1) to the dicyclo to hexacylo paraffin component (R2) as defined in the requirement (IV) which is adjusted to 0.70 or less by a volume ratio is obtained, it is possible to maintain a high viscosity index, improve low-temperature viscosity characteristics, and further improve detergency.

From the viewpoint, in the mineral base oil of one embodiment of the present invention, the content ratio [R1/R2] of the monocyclo paraffin content (R1) and the dicyclo to hexacyclo paraffin content (R2) is preferably 0.66 or less, more preferably 0.62 or less, and even more preferably 0.60 or less by a volume ratio as measured in accordance with ASTM D 2786.

In addition, the content ratio [R1/R2] is typically 0.01 or more, but from the viewpoint of obtaining a mineral base oil from which a lubricating oil composition with further improved piston detergency is easily prepared, the content ratio [R1/R2] is preferably 0.10 or more, more preferably 0.30 or more, even more preferably 0.48 or more, and still even more preferably 0.50 or more.

In the mineral base oil of one embodiment of the present invention, from the viewpoint of improving low-temperature viscosity characteristics and detergency, the content of the dicyclo to hexacyclo paraffin component (R2) based on 100 vol % of the total amount of the paraffin component in the mineral base oil is preferably 15 to 70 vol %, more preferably 17 to 60 vol %, even more preferably 20 to 50 vol %, still more preferably 23 to 40 vol %, and particularly preferably 23 to 30 vol %, as measured in accordance with ASTM D 2786.

In the mineral base oil of one embodiment of the present invention, the content of the monocyclo paraffin component (R1) is preferably 3 to 29 vol %, more preferably 5 to 25 vol %, even more preferably 7 to 20 vol %, and still more preferably 10 to 18 vol % based on 100 vol % of the total amount of the paraffin component in the mineral base oil, as measured with accordance with ASTM D2786.

In the mineral base oil of one embodiment of the present invention, the content of an acyclic paraffin component (RO) is preferably 1 to 80 vol %, more preferably 10 to 75 vol %, even more preferably 20 to 70 vol %, and still more preferably 30 to 65 vol % based on 100 vol % of the total amount of the paraffin component in the mineral base oil, as measured with accordance with ASTM D2786.

In the present specification, the aforementioned “contents of the components (RO) to (R2) based on 100 vol % of the total amount of the paraffin component in the mineral base oil” are values calculated as described below.

-   -   Procedure (1): The contents of the components (RO) to (112) are         respectively determined based on the measurement method in         accordance with ASTM D2786.     -   Procedure (2): The total content of the components (RO), (R1)         and (R2) determined in Procedure (1) is defined as “the total         amount of the paraffin component”.     -   Procedure (3): The contents of the components (RO) to (R2) are         respectively calculated based on 100 vol % of the total content         (total amount of the paraffin portion) of the components (RO),         (R1) and (R2).

Furthermore, according to the studies of the present inventors, it was found that in the mineral base oil satisfying the requirement (III), an action of dissolving coking which might occur under a high temperature environment was more expressed by preparing a mineral base oil in which the content of the cyclic paraffin component (naphthene component) was adjusted to a larger value.

For that reason, a lubricating oil composition having a more improved high-temperature detergency of a piston may also be produced by using the mineral base oil of the present invention.

From the viewpoint, in the mineral base oil of one embodiment of the present invention, the total content of the monocyclo paraffin component (R1) and the dicyclo to hexacyclo paraffin component (R2) is preferably 20 vol % or more, more preferably 25 vol % or more, even more preferably 30 vol % or more, still even more preferably 35 vol %, and particularly preferably 40 vol % or more, and is typically less than 100 vol %, preferably 99 vol % or less, more preferably 90 vol % or less, even more preferably 80 vol % or less, and still even more preferably 70 vol % or less based on 100 vol % of the total amount of the paraffin component in the mineral base oil, as measured in accordance with ASTM D2786.

Further, from the viewpoint, a naphthene content (% C_(N)) of the mineral base oil of one embodiment of the present invention is preferably 15 to 30, more preferably 16 to 30, even more preferably 18 to 30, and still even more preferably 20 to 30.

<Requirement (V)>

The mineral base oil of the present invention needs to have an aromatic content (% C_(A)) of less than 1.0 as defined in the requirement (V). A lubricating oil composition containing a mineral base oil prepared so as to have an aromatic content (% C_(A)) adjusted to less than 1.0 may be excellent in high-temperature detergency of a piston.

From the viewpoint, the aromatic content (% C_(A)) of the mineral base oil of one embodiment of the present invention is preferably 0.1 or less, and more preferably 0.01 or less.

In addition, from the viewpoint of obtaining a mineral base oil capable of producing a lubricating oil composition which is excellent in high-temperature detergency of a piston, it is preferred for the mineral base oil of one embodiment of the present invention satisfying the requirement (V) to have a lower content of a sulfur component.

In the specific mineral base oil of one embodiment of the present invention, the content of the sulfur component is preferably less than 500 ppm by mass, and more preferably less than 100 ppm by mass, based on the total amount (100 mass %) of the mineral base oil.

Furthermore, in the present specification, the content of the sulfur component in the mineral base oil is a value measured in accordance with JIS K 2541-6:2003, “Crude Oil and Petroleum Product—Sulfur Component Test Method.”

<Requirement (VI)>

The requirement (VI) is one of the indices showing the low-temperature viscosity characteristics of a mineral base oil under a low temperature environment, which is independent of the requirement (III).

A mineral base oil with a low complex viscosity η* at −35° C. as prescribed by the requirement (VI) tends to have a low paraffin content. Accordingly, by using such a mineral base oil, a lubricating oil composition having desirable low-temperature viscosity characteristics, including low-temperature fuel consumption and low-temperature engine start-up performance, and improved high-temperature piston detergency can be produced.

From the viewpoint, in the mineral base oil of one embodiment of the present invention, the complex viscosity η* at −35° C., which is defined in the requirement (VI), is preferably 60,000 Pa·s or less, but more preferably 40,000 Pa·s or less, even more preferably 10,000 Pa·s or less, still even more preferably 6,500 Pa·s or less, further preferably 6,000 Pa·s or less, still further preferably 2,000 Pa·s or less, and particularly preferably 500 Pa·s or less.

Though a lower limit value of the complex viscosity η* at −35° C. as prescribed by the requirement (VI) is not particularly limited, it is preferably 0.1 Pa·s/° C. or more, more preferably 1 Pa·s/° C. or more, and still more preferably 2 Pa·s/° C. or more.

Preparation Example of Mineral Base Oil Satisfying Requirements (I) to (VI)

The mineral base oil satisfying the requirements (I) to (VI), particularly the requirements (III) and (VI) can be easily prepared by appropriately considering, for example, the following matters. The following matters merely represent an example of the preparation method, and it is also possible to prepare the mineral base oil by considering matters different from the following matters.

(1) Adjustment of Weight Average Molecular Weight of Mineral Base Oil

The weight average molecular weight (Mw) of the mineral base oil is a physical property that affects the properties as prescribed by the requirements (I) to (VI) (particularly, the properties as prescribed by the requirements (III) and (IV)).

From the viewpoint of producing a mineral base oil satisfying the requirements (I) to (VI), particularly the requirements (I), (III), and (IV), a weight average molecular weight (Mw) of the mineral base oil of one embodiment of the present invention is preferably 450 or less, and it is preferably 150 or more.

(2) Selection of Feedstock Oil as Feedstock of Mineral Base Oil

The mineral base oil of one embodiment of the present invention is preferably one obtained by purifying a feedstock oil.

From the viewpoint of producing a mineral base oil satisfying the requirements (I) to (VI), particularly the requirements (III), (IV), and (VI), the feedstock oil is preferably a feedstock oil containing a petroleum-derived wax, or a feedstock oil containing a wax and a bottom oil. In addition, a feedstock oil containing a solvent dewaxed oil may also be used.

In the case of using a feedstock oil containing a wax and a bottom oil, from the viewpoint of producing a mineral base oil satisfying the requirements (III), (IV), and (VI), a content ratio of the wax and the bottom oil [wax/bottom oil] in the feedstock oil is preferably 30/70 to 95/5, more preferably 55/45 to 95/5, still more preferably 70/30 to 95/5, and yet still more preferably 75/25 to 95/5 in terms of a mass ratio.

As the proportion of the bottom oil in the feedstock oil increases, the value of the temperature gradient Δ|η*| of complex viscosity as prescribed by requirement (III) tends to increase, and the value of the complex viscosity η* at −35° C. as prescribed by the requirement (VI) is also liable to increase.

Meanwhile, since a large amount of a cyclic paraffin component (naphthene component)(particularly, a dicyclo to hexacyclo paraffin content (R2)) is contained in the bottom oil, a mineral base oil satisfying the requirement (IV) is easily prepared by using a feedstock oil containing a bottom oil. Further, simultaneously, it is possible to prepare a mineral base oil having a high cyclic paraffin component (naphthene component), so that a lubricating oil composition using the resulting mineral base oil may be good in high-temperature detergency of a piston.

As the bottom oil, there is exemplified a bottom fraction remained after hydrocracking of an oil including a heavy fuel oil obtained from a vacuum distillation unit in a common fuel oil producing process using a crude oil as a feedstock, followed by separation and removal of naphtha and a kerosene-gas oil.

In addition, examples of the wax include, in addition to a wax separated by solvent dewaxing of the aforementioned bottom fraction, a wax obtained by subjecting a crude oil, such as a paraffin-based oil, an intermediate-based crude oil, and a naphthalene-based crude oil to atmospheric distillation, and separating and removing naphtha and light kerosene, followed by solvent dewaxing of the remaining atmospheric bottom oil; a wax obtained by solvent dewaxing of a distillate obtained by subjecting the atmospheric bottom oil to distillation under reduced pressure; and a wax obtained by solvent dewaxing of a product obtained by subjecting the distillate to solvent deasphalting, solvent extraction, and hydrogenation process.

In addition, examples of the wax include a GTL wax obtained by Fischer Tropsch synthesis using natural gas as a feedstock.

On the other hand, as the solvent dewaxed oil, there is exemplified a residue after solvent dewaxing of the aforementioned bottom fraction or the like, followed by separation and removal of the aforementioned wax. In addition, the solvent dewaxed oil is one having been subjected to a purification process by solvent dewaxing and is different from the aforementioned bottom oil.

The method for obtaining a wax through solvent dewaxing is preferably a method in which, for example, the bottom fraction is mixed with a mixed solvent of methyl ethyl ketone and toluene, and the precipitate is removed while agitating the mixture in a low temperature region.

From the viewpoint of producing a mineral base oil satisfying the requirements (III) and (VI), a specific temperature in the solvent dewaxing in a low-temperature environment is preferably lower than the typical solvent dewaxing temperature. Specifically, the temperature is preferably −25° C. or lower, and more preferably −30° C. or lower.

From the viewpoint of producing a mineral base oil satisfying the requirements (III), (IV), and (VI), the content of an oil component of the feedstock oil is preferably 5 to 55% by mass, more preferably 7 to 45% by mass, still more preferably 10 to 35% by mass, yet still more preferably 15 to 32% by mass, and especially preferably 21 to 30% by mass.

From the viewpoint of producing a mineral base oil satisfying the requirement (I), the kinematic viscosity at 100° C. of the feedstock oil is preferably 2.0 to 7.0 mm²/s, more preferably 2.3 to 6.5 mm²/s, and still more preferably 2.5 to 6.0 mm²/s.

From the viewpoint of producing a mineral base oil satisfying the requirement (II), the viscosity index of the feedstock oil is preferably 100 or more, more preferably 110 or more, and still more preferably 120 or more.

(3) Setting of Purification Conditions for Feedstock Oil

Preferably, the feedstock oil is subjected to a purification process to prepare a mineral base oil satisfying the requirements (I) to (VI).

Preferably, the purification process includes at least one of a hydrogenation isomerization dewaxing process and a hydrogenation process. Preferably, the type of the purification process and the purification conditions are appropriately set according to the kind of the feedstock oil to be used.

More specifically, from the viewpoint of producing a mineral base oil satisfying the requirements (III) to (VI), it is preferred to select a purification process according to the kind of the feedstock oil to be used in the following manner.

In the case of using a feedstock oil (a) containing a wax and a bottom oil in the foregoing content ratio, it is preferred that the feedstock oil (a) is subjected to a purification process including both a hydrogenation isomerization dewaxing process and a hydrogenation process.

In the case of using a feedstock oil (β) containing a solvent dewaxed oil, it is preferred that the feedstock oil (β) is subjected to a purification process including a hydrogenation process without performing a hydrogenation isomerization dewaxing process.

The hydroisomerization dewaxing process can provide a mineral base oil satisfying, particularly, the requirements (III) and (VI) by converting the straight-chain paraffin in the wax into the branched-chain isoparaffin.

In addition, the feedstock oil (α) contains a bottom oil, and therefore, the contents of aromatic, sulfur, and nitrogen components tend to increase. The presence of the aromatic, sulfur, and nitrogen components becomes a factor that generates a deposit in a lubricating oil composition and causes a lowering of the high-temperature piston detergency performance.

Accordingly, it is possible to remove the aromatic component, the sulfur component, and the nitrogen component by the hydrogenation process, and to reduce the contents thereof.

In addition, since the aromatic component is converted to a paraffin component by being ring-opened by the hydrogenation process, the aromatic content (% C_(A)) may be decreased, thereby obtaining a mineral base oil satisfying the requirement (V).

On the other hand, though the feedstock oil (β) contains a wax, the straight-chain paraffin is separated and removed through precipitation in a low-temperature environment in a solvent dewaxing process, and therefore, the content of the straight-chain paraffin that affects the value of the complex viscosity value as prescribed by the requirements (III) and (VI) is small. Accordingly, there is less need to perform the “hydrogenation isomerization dewaxing process.”

(Hydroisomerization Dewaxing Process)

The hydrogenation isomerization dewaxing process is a purification process that is performed for purposes of isomerizing the straight-chain paraffin contained in the feedstock oil into a branched-chain isoparaffin, and so on, as described above.

In particular, the presence of the straight-chain paraffin is one of factors that increase the value of the temperature gradient Δ|η*| of complex viscosity prescribed by requirement (III). Therefore, according to this process, the value of the temperature gradient Δ|η*| of complex viscosity is adjusted low through isomerization of the straight-chain paraffin into a branched-chain isoparaffin.

Preferably, the hydrogenation isomerization dewaxing process is performed in the presence of a hydrogenation isomerization dewaxing catalyst.

Examples of the hydrogenation isomerization dewaxing catalyst include catalysts with a metal oxide of nickel (Ni)/tungsten (W), nickel (Ni)/molybdenum (Mo), cobalt (Co)/molybdenum (Mo), etc., or a noble metal, such as platinum (Pt), lead (Pd), etc., supported on a carrier, such as silicoaluminophosphate (SAPO), zeolite, etc.

From the viewpoint of producing a mineral base oil satisfying the requirements (III) to (VI) (particularly, the requirements (III) and (VI)), a hydrogen partial pressure in the hydrogenation isomerization dewaxing process is preferably 2.0 to 220 MPa, more preferably 2.5 to 100 MPa, still more preferably 3.0 to 50 MPa, and yet still more preferably 3.5 to 25 MPa.

From the viewpoint of producing a mineral base oil satisfying the requirements (III) to (VI) (particularly, the requirements (III) and (VI)), a reaction temperature in the hydrogenation isomerization dewaxing process is preferably set to a temperature higher than the reaction temperature of a common hydrogenation isomerization dewaxing process, and specifically, it is preferably 320 to 480° C., more preferably 325 to 420° C., still more preferably 330 to 400° C., even more preferably 330 to 370° C., and yet even more preferably 335 to 360° C.

When the reaction temperature is a high temperature, the isomerization of the straight-chain paraffin existent in the feedstock oil into a branched-chain isoparaffin can be promoted, whereby it becomes easy to prepare a mineral base oil satisfying the requirements (III) to (VI) (particularly, the requirements (III) and (VI)).

From the viewpoint of producing a mineral base oil satisfying the requirements (III) to (VI) (particularly, the requirements (III) and (VI)), a liquid hourly space velocity (LHSV) in the hydrogenation isomerization dewaxing process is preferably 5.0 hr⁻¹ or less, more preferably 2.0 hr⁻¹ or less, still more preferably 1.3 hr⁻¹ or less, and yet still more preferably 1.0 hr⁻¹ or less.

From the viewpoint of improving the productivity, the LHSV in the hydrogenation isomerization dewaxing process is preferably 0.1 hr⁻¹ or more, and more preferably 0.2 hr⁻¹ or more.

A supply proportion of the hydrogen gas in the hydrogenation isomerization dewaxing process is preferably 100 to 1,000 Nm³, more preferably 200 to 800 Nm³, and still more preferably 250 to 650 Nm³ per kiloliter of the feedstock oil to be supplied.

The generated oil after the hydrogenation isomerization dewaxing process may be subjected to vacuum distillation for the purpose of removing the light fraction.

(Hydrogenation Process)

The hydrogenation process is a purification process that is performed for purposes of complete saturation of the aromatic component contained in the feedstock oil, removal of impurities, such as the sulfur component, the nitrogen component, etc., and so on.

Preferably, the hydrogenation process is performed in the presence of a hydrogenation catalyst.

Examples of the hydrogenation catalyst include catalysts with a metal oxide of nickel (Ni)/tungsten (W), nickel (Ni)/molybdenum (Mo), cobalt (Co)/molybdenum (Mo), etc., or a noble metal, such as platinum (Pt), lead (Pd), etc., supported on an amorphous carrier, such as silica/alumina, alumina, etc., or a crystalline carrier, such as zeolite, etc.

From the viewpoint of producing a mineral base oil satisfying the requirements (III) to (VI) (particularly, the requirements (III) and (VI)), a hydrogen partial pressure in the hydrogenation process is preferably set to a pressure higher than the pressure of a common hydrogenation process, and specifically, it is preferably 16 MPa or more, more preferably 17 MPa or more, and still more preferably 20 MPa or more, and it is preferably 30 MPa or less, and more preferably 22 MPa or less.

From the viewpoint of producing a mineral base oil satisfying the requirements (III) to (VI) (particularly, the requirements (III) and (VI)), a reaction temperature in the hydrogenation process is preferably 200 to 400° C., more preferably 250 to 350° C., and still more preferably 280 to 330° C.

From the viewpoint of producing a mineral base oil satisfying the requirements (III) to (VI) (particularly, the requirements (III) and (VI)), a liquid hourly space velocity (LHSV) in the hydrogenation process is preferably 5.0 hr⁻¹ or less, more preferably 2.0 hr⁻¹ or less, and still more preferably 1.3 hr⁻¹ or less, and yet still more preferably 1.0 hr⁻¹ or less, and from the viewpoint of productivity, it is preferably 0.1 hr⁻¹ or more, more preferably 0.2 hr⁻¹ or more, and still more preferably 0.3 hr⁻¹ or more.

A supply proportion of the hydrogen gas in the hydrogenation process is preferably 100 to 1,000 Nm³, more preferably 200 to 800 Nm³, and still more preferably 250 to 650 Nm³ per kiloliter of the supplied oil as a processing object.

The generated oil after the hydrogenation process may be subjected to vacuum distillation for the purpose of removing the light fraction. Various conditions of the vacuum distillation (e.g., pressure, temperature, time, etc.) are appropriately adjusted so as to make the kinematic viscosity at 100° C. of the mineral base oil fall within a desirable range.

<Various Physical Properties of Mineral Base Oil>

A CCS viscosity (low-temperature viscosity) at −35° C. of the mineral base oil of one embodiment of the present invention is preferably 5,000 mPa·s or less, more preferably 4,000 mPa·s or less, even more preferably 3,000 mPa·s or less, still even more preferably 2,700 mPa·s or less, and particularly preferably 2,500 mPa·s or less.

Preferably, the mineral base oil of one embodiment of the present invention satisfies the following requirement (P-1).

-   -   Requirement (P-1): the percentage increase P of the deposit         amount (W) is 12% or less with respect to the amount (W₀) of the         deposit measured based on a panel coking test under the         conditions described in the examples using the mineral base oil         as a sample, as measured based on the same panel coking test by         using a lubricating oil composition obtained by blending 1.00         mass % of a pour-point depressant with the mineral base oil, as         a sample.

The percentage increase P of the mass (W) of the deposit may also be expressed as “P (unit: %)=(W−W₀)/W₀×100.”

Since the mineral base oil of the present invention satisfies the requirement (III), it is possible to not only provide a lubricating oil composition with good low-temperature viscosity characteristics, but also maintain favorably the high-temperature detergency of a piston of the lubricating oil composition even as a lubricating oil composition obtained by blending a polymer component which causes detergency to deteriorate together with the mineral base oil.

The percentage increase P defined in the requirement (P-1) becomes an index of the high-temperature detergency of the piston of the resulting lubricating oil composition when a target mineral base oil is blended with the polymer component, so that the percentage increase P is more preferable as the value increases.

The percentage increase P (%) of the deposit amount (W) is preferably 10% or less, more preferably 8.5% or less, and still more preferably 8% or less.

[Lubricating Oil Composition]

The lubricating oil composition of the present invention contains at least the above-described mineral base oil of the present invention, but may contain a synthetic oil together with the mineral base oil of the present invention.

Examples of the synthetic oil include a poly α-olefin (PAO), an ester-based compound, an ether-based compound, polyglycol, alkylbenzene, and alkylnaphthalene.

These synthetic oils may be used either alone or in combination of two or more thereof.

The content of the synthetic oil in the lubricating oil composition of the present invention is preferably 0 to 30 parts by mass, more preferably 0 to 20 parts by mass, even more preferably 0 to 15 parts by mass, still even more preferably 0 to 10 parts by mass, and particularly preferably 0 to 5 parts by mass based on 100 parts by mass of the total amount of the mineral base oil of the present invention in the lubricating oil composition.

The content of the mineral base oil of the present invention contained in the lubricating oil composition of one embodiment of the present invention is typically 50 mass % or more, preferably 55 mass % or more, more preferably 60 mass % or more, even more preferably 65 mass % or more, still even more preferably 70 mass % or more, and particularly preferably 80 mass % or more, and is preferably 100 mass % or less, more preferably 99 mass % or less, and even more preferably 95 mass % or less, based on the total amount (100 mass %) of the lubricating oil composition.

Further, the lubricating oil composition of the present invention may contain an additive for lubricating oil, which is more generally used, if necessary, within a range not impairing the effect of the present invention.

Examples of the additive for lubricating oil include a pour-point depressant, a viscosity index improver, a metal-based detergent, a dispersant, an anti-wear agent, an extreme-pressure agent, an antioxidant, an anti-foaming agent, a friction modifier, a rust inhibitor, and a metal deactivator.

In addition, as the additive for lubricating oil, a commercially available additive package containing a plurality of additives complying with the API/ILSAC SN/GF-5 standard or the like may be used.

Further, a compound having a plurality of functions as the aforementioned additive (for example, a compound having a function as an anti-wear agent and an extreme-pressure agent) may be used.

Furthermore, the respective additives for lubricating oil may be used either alone or in combination of two or more thereof.

The content of each of these additives for lubricating oil can be appropriately adjusted within a range not impairing the effect of the present invention, but the content is typically 0.001 to 15 mass %, preferably 0.005 to 10 mass %, and more preferably 0.01 to 8 mass % based on the total amount (100 mass %) of the lubricating oil composition.

Further, in the lubricating oil composition of one embodiment of the present invention, the total content of these additives for lubricating oil is preferably 0 to 30 mass %, more preferably 0 to 25 mass %, even more preferably 0 to 20 mass %, and still more preferably 0 to 15 mass % based on the total amount (100 mass %) of the lubricating oil composition.

(Pour-Point Depressant)

Examples of the pour-point depressant include an ethylene-vinyl acetate copolymer, a condensate of chlorinated paraffin and naphthalene, a condensate of chlorinated paraffin and phenol, polymethacrylate, and polyalkyl styrene, and polymethacrylate is preferably used.

(Viscosity Index Improver)

Examples of the viscosity index improver include a polymer, such as a non-dispersant-type polymethacrylate, a dispersant-type polymethacrylate, an olefin-based copolymer (for example, an ethylene-propylene copolymer, and the like), a dispersant-type olefin-based copolymer, and a styrene-based copolymer (for example, a styrene-diene copolymer, a styrene-isoprene copolymer, and the like).

The weight average molecular weight (Mw) of these viscosity index improvers is typically 500 to 1,000,000, preferably 5,000 to 800,000, and more preferably 10,000 to 600,000, but the weight average molecular weight is set appropriately according to the type of polymer.

In addition, in the non-dispersant-type and dispersant-type polymethacrylates used as the viscosity index improver, the weight average molecular weight thereof is preferably 5,000 to 1,000,000, more preferably 10,000 to 800,000, and even more preferably 20,000 to 500,000.

Furthermore, in the olefin-based copolymer used as the viscosity index improver, the weight average molecular weight thereof is preferably 800 to 1,000,000, more preferably 1,000 to 800,000, and even more preferably 10,000 to 600,000.

(Metal-Based Detergent)

Examples of the metal-based detergent include an organic acid metal salt compound containing a metal atom selected from alkali metals and alkaline earth metals, and specific examples thereof include a metal salicylate, a metal phenate, and a metal sulfonate, containing a metal atom selected from alkali metals and alkaline earth metals.

In this specification, the “alkali metal” refers to lithium, sodium, potassium, rubidium, cesium, or francium.

In addition, the “alkaline earth metal” refers to beryllium, magnesium, calcium, strontium, and barium.

From the viewpoint of improving the high-temperature detergency, the metal atom to be contained in the metal-based detergent is preferably sodium, calcium, magnesium, or barium, and more preferably calcium.

The metal salicylate is preferably a compound represented by the following general formula (1); the metal phenate is preferably a compound represented by the following general formula (2); and the metal sulfonate is preferably a compound represented by the following general formula (3).

In the general formulae (1) to (3), M is a metal atom selected from alkali metals and alkaline earth metals, sodium, calcium, magnesium, or barium is preferred, and calcium is more preferred. Furthermore, M′ is an alkaline earth metal, calcium, magnesium, or barium is preferred, and calcium is more preferred. p is the valence of M and is 1 or 2. R is a hydrogen atom or a hydrocarbon group having 1 to 18 carbon atoms. q is an integer of 0 or more, preferably an integer of 0 to 3.

Examples of the hydrocarbon group which may be selected as R include an alkyl group having 1 to 18 carbon atoms, an alkenyl group having 1 to 18 carbon atoms, a cycloalkyl group having 3 to 18 ring carbon atoms, an aryl group having 6 to 18 ring carbon atoms, an alkylaryl group having 7 to 18 carbon atoms, and an arylalkyl group having 7 to 18 carbon atoms.

In one embodiment of the present invention, these metal-based detergents may be used either alone or in combination of two or more thereof.

Among these, from the viewpoints of an improvement in the high-temperature detergency and solubility in the base oil, the metal-based detergent is preferably at least one selected from calcium salicylate, calcium phenate, and calcium sulfonate.

In one embodiment of the present invention, the metal-based detergent may be any of a neutral salt, a basic salt, an overbased salt, and a mixture thereof.

The total base number of the metal-based detergent is preferably 0 to 600 mgKOH/g.

In one embodiment of the present invention, in the case where the metal-based detergent is a basic salt or an overbased salt, the total base number of the metallic detergency is preferably 10 to 600 mgKOH/g, and more preferably 20 to 500 mgKOH/g.

In this specification, the “base number” means a base number measured by the perchloric acid method in accordance with Item 7 of the “Petroleum Products and Lubricants—Determonation of neutralization number” of JIS K2501.

(Dispersant)

Examples of the dispersant include succinimide, benzylamine, succinate, or a boron modified product thereof.

Examples of the succinimide include a monoimide or bisimide of a succinic acid having a polyalkenyl group, such as a polybutenyl group having a number average molecular weight of 300 to 4,000 and a polyethylene polyamine, such as ethylenediamine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, and pentaethylenehexamine, or a boron modified product thereof, and a Mannich reaction product of a phenol having a polyalkenyl group, formaldehyde, and polyethylene polyamine.

(Anti-Wear Agent)

Examples of the anti-wear agent include sulfur-containing compounds, such as zinc dialkyl dithiophosphate (ZnDTP), zinc phosphate, zinc dithiocarbamate, molybdenum dithiocarbamate, molybdenum dithiophosphate, disulfides, sulfurized olefins, sulfurized oils and fats, sulfurized esters, thiocarbonates, thiocarbamates, and polysulfides; phosphorus-containing compounds, such as phosphite esters, phosphate esters, phosphonate esters, and amine salts or metal salts thereof; and sulfur- and phosphorous-containing anti-wear agents, such as thiophosphite esters, thiophosphate esters, thiophosphonate esters, and amine salts or metal salts thereof.

Among these, a zinc dialkyl dithiophosphate (ZnDTP) is preferred, and a combination of a primary alkyl-type zinc dialkyl dithiophosphate and a secondary alkyl-type zinc dialkyl dithiophosphate is more preferred.

(Extreme-Pressure Agent)

Examples of the extreme-pressure agent include a sulfur-based extreme-pressure agent such as sulfides, sulfoxides, sulfones, and thiophosphinates, a halogen-based extreme-pressure agent such as chlorinated hydrocarbons, and an organic metal-based extreme-pressure agent. Further, among the above-described anti-wear agents, a compound having a function as an extreme-pressure agent can be used.

In one embodiment of the present invention, these extreme-pressure agents may be used either alone or in combination of two or more thereof.

(Antioxidant)

As the antioxidant, any publicly-known antioxidant can be appropriately selected and used among publicly-known antioxidants used in the related art as an antioxidant for lubricating oil, and examples thereof include an amine-based antioxidant, a phenol-based antioxidant, a molybdenum-based antioxidant, a sulfur-based antioxidant, and a phosphorus-based antioxidant.

Examples of the amine-based antioxidant include a diphenylamine-based antioxidant, such as diphenylamine and an alkylated diphenylamine having an alkyl group having 3 to 20 carbon atoms; and a naphthylamine-based antioxidant, such as α-naphthylamine, phenyl-α-naphthylamine, and a substituted phenyl-α-naphthylamine having an alkyl group having 3 to 20 carbon atoms.

Examples of the phenol-based antioxidant include a monophenol-based antioxidant, such as 2,6-di-tert-butylphenol, 2,6-di-tert-butyl-4-methylphenol, 2,6-di-tert-butyl-4-ethylphenol, isooctyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, and octadecyl-3-(3, 5-di-tert-butyl-4-hydroxyphenyl)propionate; a diphenol-based antioxidant, such as 4,4′-methylenebis(2,6-di-tert-butylphenol) and 2,2′-methylenebis(4-ethyl-6-tert-butylphenol); and a hindered phenol-based antioxidant.

Examples of the molybdenum-based antioxidant include a molybdenum amine complex obtained by allowing molybdenum trioxide and/or molybdic acid to react with an amine compound.

Examples of the sulfur-based antioxidant include dilauryl-3,3′-thiodipropionate.

Examples of the phosphorus-based antioxidant include phosphite.

In one embodiment of the present invention, these antioxidants may be used either alone or in combination of two or more thereof, but it is preferred that two or more thereof are used in combination.

(Anti-Foaming Agent)

Examples of the anti-foaming agent include silicone oil, fluorosilicone oil, and fluoroalkyl ether.

(Friction Modifier)

Examples of the friction modifier include a molybdenum-based friction modifier, such as molybdenum dithiocarbamate (MoDTC), molybdenum dithiophosphate (MoDTP), and an amine salt of molybdic acid; an ash-free friction modifier having at least one alkyl group or alkenyl group having 6 to 30 carbon atoms, such as an aliphatic amine, a fatty acid ester, a fatty acid amide, a fatty acid, an aliphatic alcohol, and an aliphatic ether; oils and fats, amines, amides, sulfurized esters, phosphate esters, phosphite esters, and phosphate ester amine salts.

(Rust Inhibitor)

Examples of the rust inhibitor include fatty acid, alkenyl succinic half ester, fatty acid soap, alkyl sulfonate, polyhydric alcohol fatty acid ester, fatty acid amine, oxidized paraffin, and alkyl polyoxyethylene ether.

(Metal Deactivator)

Examples of the metal deactivator include a benzotriazole-based compound, a tolyltriazole-based compound, a thiadiazole-based compound, an imidazole-based compound, and a pyrimidine-based compound.

In one embodiment of the present invention, these metal deactivators may be used either alone or in combination of two or more thereof.

<Embodiment of Lubricating Oil Composition of Present Invention>

Examples of the lubricating oil composition of one embodiment of the present invention include a lubricating oil composition (X-1) containing a base oil including the above-described mineral base oil of the present invention and having a content of an additive for lubricating oil, which is composed of a polymer component, of 10 mass % or less.

The polymer component contained as an additive for lubricating oil is a component that causes coking which causes deterioration in the high-temperature detergency of a piston, and means a compound having at least one repeating unit.

However, since the lubricating oil composition (X-1) uses a mineral base oil satisfying the requirement (III) as a base oil, even though coking occurs from the polymer component, it is possible to maintain high-temperature detergency of the piston favorably by dissolving the coking.

Examples of the polymer component that causes coking include a viscosity index improver or a pour-point depressant, and include particularly polymethacrylate contained as a viscosity index improver or a pour-point depressant.

In addition, the weight average molecular weight of the polymer component which causes coking is typically 100,000 or less.

In the lubricating oil composition (X-1), the content of the polymer component which causes such coking is adjusted.

Examples of a specific embodiment of the lubricating oil composition (X-1) include a lubricating oil composition (X-11) containing a pour-point depressant in an amount 10 mass % or less, a lubricating oil composition (X-12) containing a viscosity index improver in an amount of 10 mass % or less, a lubricating oil composition (X-13) having a polymethacrylate in an amount of 10 mass % or less, and a lubricating oil composition (X-14) containing a polymer component having a weight average molecular weight of 100,000 or less in an amount of 10 mass % or less.

In the lubricating oil composition (X-1), the content of the additive for lubricating oil, consisting of a polymer component is preferably 0.01 to 10 mass %, more preferably 0.03 to 7 mass %, even more preferably 0.05 to 5 mass %, and still more preferably 0.1 to 3 mass %, based on the total amount (100 mass %) of the lubricating oil composition (X-1).

In addition, examples of the lubricating oil composition of another embodiment of the present invention include a lubricating oil composition (Y-1) which contains a base oil including the above-described mineral base oil of the present invention and substantially does not contain the above-described polymer component.

Here, the term “substantially does not contain a polymer component” means that the content of the polymer component is less than 0.01 mass % (preferably less than 0.001 mass %, and more preferably 0 mass % (not detected)), based on the total amount (100 mass %) of a lubricating oil composition (B).

Since the Lubricating Oil Composition (Y-1) substantially does not contain the above-described polymer component, which causes deterioration in high-temperature detergency of a piston, the lubricating oil composition (Y-1) has excellent piston detergency.

Examples of a specific embodiment of the lubricating oil composition (Y-1) include a lubricating oil composition (Y-11) which substantially does not contain a pour-point depressant, a lubricating oil composition (Y-12) which substantially does not contain a viscosity index improver, a lubricating oil composition (Y-13) which substantially does not contain polymethacrylate, and a lubricating oil composition (Y-14) which substantially does not contain a polymer component having a weight average molecular weight of 100,000 or less.

<Method of Producing Lubricating Oil Composition>

A method of producing the lubricating oil composition of the present invention, there is no particular limitation, but as a method for producing a lubricating oil composition containing the above-described additive for lubricating oil, a method including a process of blending the additive for lubricating oil with a base oil containing the mineral base oil of the present invention is preferred.

In the process, preferred compounds of each additive for lubricating oil to be blended or the content of each component is as described above.

Preferably, an additive for lubricating oil is blended with the base oil containing the mineral base oil of the present invention, and then the additive for lubricating oil is uniformly dispersed in the base oil by a publicly-known method by stirring the blend.

In addition, from the viewpoint of uniformly dispersing the additive for lubricating oil, it is more preferred that the base oil containing the mineral base oil of the present invention is heated to 40 to 70° C., and then an additive for lubricating oil is blended with the base oil, and is uniformly dispersed by stirring the blend.

A lubricating oil composition obtained by blending an additive for lubricating oil with the base oil containing the mineral base oil of the present invention, and then modifying a portion of the base oil or the additive for lubricating oil or by allowing two components to react with each other, and producing a separate component corresponds to a lubricating oil composition obtained by the method of producing a lubricating oil composition of the present invention, and thus falls within a technical scope of the present invention.

<Various Physical Properties of Lubricating Oil Composition>

The kinematic viscosity of the lubricating oil composition of one embodiment of the present invention at 100° C. is preferably 4 mm²/s or more, more preferably 5 mm²/s or more, even more preferably 6 mm²/s or more, and still more preferably 7 mm²/s or more, and also is preferably less than 15 mm²/s, more preferably less than 12.5 mm²/s, even more preferably less than 11 mm²/s, and still even more preferably less than 10 mm²/s.

The viscosity index of the lubricating oil composition of one embodiment of the present invention is preferably 140 or more, more preferably 150 or more, and even more preferably 160 or more.

The temperature gradient Δ|η*| of complex viscosity between two points of −10° C. and −25° C., which is defined in the same manner as the requirement (III) of the lubricating oil composition of one embodiment of the present invention, is preferably 60 Pa·s/° C. or less, more preferably 20 Pa·s/° C. or less, even more preferably 15 Pa·s/° C. or less, still even more preferably 10 Pa·s/° C. or less, and particularly preferably 5 Pa·s/° C. or less.

Further, in the lubricating oil composition of one embodiment of the present invention, the lower limit value of the temperature gradient Δ|η*| of complex viscosity, which is defined in the same manner as the requirement (III), is not particularly limited, but is preferably 0.001 Pa·s/° C. or more, and more preferably 0.01 Pa·s/° C. or more.

The complex viscosity η* at −35° C. as similarly prescribed by the requirement (VI) of the lubricating oil composition of one embodiment of the present invention, is preferably 45,000 Pa·s or less, more preferably 35,000 Pa·s or less, even more preferably 6,000 Pa·s or less, still even more preferably 2,000 Pa·s or less, and particularly preferably 500 Pa·s or less.

In addition, in the lubricating oil composition of one embodiment of the present invention, the lower limit value of the complex viscosity η* at −35° C. as similarly prescribed by the requirement (VI), is not particularly limited, but is preferably 0.1 Pa·s or more, more preferably 1 Pa·s or more, and even more preferably 2 Pa·s or more.

From the viewpoint of obtaining a lubricating oil composition having good low-temperature viscosity characteristics, the CCS viscosity (low temperature viscosity) of the lubricating oil composition of one embodiment of the present invention at −35° C. is preferably 9,000 mPa·s or less, more preferably 8,600 mPa·s or less, even more preferably 7,500 mPa·s or less, still even more preferably 7,000 mPa·s or less, and typically 1,000 mPa·s or more.

The HTHS viscosity (high-temperature high shear viscosity) of the lubricating oil composition of one embodiment of the present invention at 150° C. is preferably 1.4 mPa·s or more and less than 3.8 mPa·s, preferably 1.4 mPa·s or more and less than 3.5 mPa·s, more preferably 1.6 mPa·s or more and less than 3.2 mPa·s, even more preferably 1.7 mPa·s or more and less than 3.0 mPa·s, and still even more preferably 2.0 mPa·s or more and less than 2.8 mPa·s.

When the HTHS viscosity at 150° C. is 1.4 mPa·s or more, the lubricating oil composition may be a lubricating oil composition with good lubricating performance. Meanwhile, when the HTHS viscosity at 150° C. is less than 3.8 mPa·s, it is possible to obtain a lubricating oil composition with good fuel saving performance by suppressing the deterioration in viscosity characteristics at low temperature.

The aforementioned HTHS viscosity at 150° C. can also be assumed as the viscosity under a high-temperature region during high-speed operation of an engine. That is, when the HTHS viscosity of the lubricating oil composition at 150° C. falls within the above range, it can be said that the lubricating oil composition has various good properties, such as viscosity under a high temperature range assuming high speed operation of an engine.

The HTHS viscosity of the aforementioned lubricating oil composition at 150° C. is a value measured in accordance with ASTM D4741, and more specifically, means a value measured by the method described in the Examples.

In one embodiment of the present invention, a lubricating oil composition having a kinematic viscosity of less than 12.5 mm²/s at 100° C. and an HTHS viscosity of less than 3.5 mPa·s at 150° C. is preferred.

By satisfying the above requirement, the lubricating oil composition can reduce fluid friction and improve fuel saving performance.

The density of the lubricating oil composition of one embodiment of the present invention at 15° C. is preferably 0.80 to 0.90 g/cm³, and more preferably 0.82 to 0.87 g/cm³.

The density of the aforementioned lubricating oil composition at 15° C. means a value measured in accordance with JIS K 2249: 2011.

In the lubricating oil composition of one embodiment of the present invention, the deposit amount measured by the panel coking test under the conditions described in the examples is preferably less than 100 mg, more preferably less than 90 mg, even more preferably less than 85 mg, and still even more preferably less than 80 mg.

<Use of Lubricating Oil Composition>

The lubricating oil composition of the present invention has desirable low-temperature viscosity characteristics, including low-temperature fuel consumption and low-temperature engine start-up performance, and even when mixed with a polymer component as an additive, it has an excellent effect in reducing a high-temperature piston detergency drop to be caused due to the polymer component.

Accordingly, examples of engines filled with the lubricating oil composition of the present invention include engines for vehicles, such as automobiles, electric trains, aircraft, etc. Preferred are automobile engines, and more preferred are automobile engines equipped with a hybrid mechanism or a start-up system.

The lubricating oil composition of one embodiment of the present invention is suitable for uses as a lubricating oil composition for internal combustion engines of vehicles, such as automobiles, electric trains, aircraft, etc. (engine oils for internal combustion engines), and is also applicable for other uses.

Examples of the other possible use of the lubricating oil composition of one embodiment of the present invention include power steering oils, automatic transmission fluids (ATF), continuously variable transmission fluids (CVTF), hydraulic actuation oils, turbine oils, compressor oils, lubricants for machine tools, cutting oils, gear oils, fluid dynamic bearing oils, and roller bearing oils.

The lubricating oil composition of the present invention is suited for lubrication for a sliding mechanism equipped with a piston ring and a liner in a device having a sliding mechanism having a piston ring and a liner, particularly a sliding mechanism equipped with a piston ring and a liner in an internal combustion engine (preferably, an internal combustion engine of automobile).

A material for forming the piston ring or cylinder liners to which the lubricating oil composition of the present invention is applied is not particularly limited. Examples of a cylinder liner-forming material include an aluminum alloy, a cast iron alloy, and the like.

Examples of a piston ring-forming material include a Si—Cr steel, a martensite-based stainless steel containing 11 to 17% by mass of Cr, and the like. Preferably, the piston ring-forming material is subjected to a substrate treatment according to a chromium plating treatment, a chromium nitride treatment, a nitriding treatment, or a combination thereof.

[Internal Combustion Engine]

The present invention also provides an internal combustion engine having a sliding mechanism equipped with a piston ring and a liner and including the aforementioned lubricating oil composition of the present invention.

In one embodiment of the present invention, an internal combustion engine in which the lubricating oil composition of the present invention is applied to a sliding portion of the aforementioned sliding mechanism is preferred.

The lubricating oil composition of the present embodiment and the sliding mechanism equipped with a piston ring and a liner are those as described above, and as a specific configuration of the sliding mechanism, there is exemplified one shown in FIG. 2.

A sliding mechanism 1 illustrated in FIG. 2 has a block 2 having a piston travel path 2 a and a crank shaft housing 2 b, a liner 12 disposed along an inner wall of the piston travel path 2 a, a piston 4 housed in the liner 12, piston rings 6 and 8 externally fitted to the piston 4, a crank shaft 10 housed in the crank shaft housing 2 b, a connecting rod 9 connecting the crank shaft 10 and the piston 4, and a structure sandwiched between the liner 12 and the piston travel path 2 a.

The crank shaft 10 can be rotationally driven by a motor which is not illustrated, thereby reciprocating the piston 4 via the connecting rod 9.

In the sliding mechanism 1 configured as described above, the crank shaft housing 2 b is filled with the lubricating oil composition 20 of the present invention until the lubricating oil composition 20 of the present invention reaches a liquid level in a position higher than the center of the center axis of the crank shaft 10 and lower than the uppermost end of the central axis. The lubricating oil composition 20 in the crank shaft housing 2 b is supplied between the liner 12 and the piston ring 6 in a splash manner by the rotating crank shaft 10.

[Method for Lubricating Internal Combustion Engine]

The present invention also provides a method for lubricating an internal combustion engine, which lubricates an apparatus having a sliding mechanism equipped with a piston ring and a liner, in which the piston ring and the liner are lubricated by using the above-described lubricating oil composition of the present invention.

The lubricating oil composition of the present embodiment and the sliding mechanism equipped with the piston ring and the liner are as described above.

In the method of lubricating an internal combustion engine of the present invention, by using the lubricating oil composition of the present embodiment as a lubricating oil in the sliding part between the piston ring and the cylinder liner, it is possible to greatly reduce the friction and contribute to improvement in fuel economy in either the fluid lubrication or the mixed lubrication.

EXAMPLES

The present invention is hereunder described in more detail by reference to Examples, but it should be construed that the present invention is by no means limited by the following Examples. The measurement methods and evaluation methods of various physical properties are as follows.

<Method for Measuring Various Physical Properties of Mineral Base Oil or Lubricating Oil Composition>

(1) Kinematic Viscosities at 40° C. and 100° C.

Kinematic viscosities were measured in accordance with JIS K2283:2000.

(2) Viscosity Index

Viscosity index was calculated in accordance with JIS K2283:2000.

(3) CCS Viscosity at −35° C.

CCS viscosity was measured in accordance with JIS K2010:1993 (ASTM D 2602).

(4) Complex Viscosities η* at −25° C., −10° C., and −35° C.

Complex viscosities η* were measured with a rheometer, “Physica MCR 301”, manufactured by Anton Paar according to the following procedures

First of all, a mineral base oil or a lubricating oil composition to be measured was inserted in a cone plate (diameter: 50 mm, tilt angle: 1°) that had been adjusted to a measurement temperature of −25° C., −10° C., or −35° C. and then held at the same temperature for 10 minutes. On this occasion, care was taken so as not to induce a strain in the inserted solution.

Moreover, at a predetermined measurement temperature, within a range of an angular velocity of 6.3 rad/s and a strain amount of 0.1 to 100% under the conditions of values appropriately set as follows according to the measured temperature, the complex viscosity η* at each measurement temperature was measured in a vibration mode.

In the measurement of the complex viscosity η*, the strain amount was set to “0.36%” at −25° C., set to “2.1%” at −10° C., and set to “0.1%” at −35° C.

The “temperature gradient Δ|η*| of complex viscosity” was then calculated from the values of complex viscosity η* at −25° C. and −10° C. according to the aforementioned calculation formula (f1).

(5) Weight Average Molecular Weight (Mw) and Number Average Molecular Weight (Mn)

These were measured with a gel permeation chromatography device (“1260 Type HPLC”, manufactured by Agilent) under the following conditions, and the values measured as expressed in terms of a standard polystyrene conversion were adopted.

(Measurement Conditions)

-   -   Column: Two columns of “Shodex LF 404” were sequentially         connected.     -   Column temperature: 35° C.     -   Developing solvent: Chloroform     -   Flow rate: 0.3 mL/min

<Method for Measuring Various Physical Properties of Mineral Base Oil>

(6) Aromatic content (% C_(A)) and naphthene content (% C_(N))

These were measured according to the ASTM D-3238 ring analysis (n-d-M method).

(7) Content of Sulfur Component

Content of sulfur component was measured in accordance with JIS K2541-6:2003.

(8) Content of Nitrogen Component

Content of nitrogen component was measured in accordance with JIS K2609:1998 4.

(9) Type Analysis of Paraffin Content (Content of Each Component)

In accordance with ASTM D2786, the contents of an acyclic paraffin content and a monocyclo to hexacyclo paraffin component in the mineral base oil were respectively determined.

In addition, the content of the acyclic paraffin component (RO), the content of the monocyclo paraffin component (R1), and the content of the dicyclo to hexacyclo paraffin component (R2) were respectively calculated based on 100 vol % of the total amount of a paraffin component including an acyclic paraffin component and a monocyclo to hexacyclo paraffin component. Furthermore, the content ratio [R1/R2] (volume ratio) of the monocyclo paraffin component (R1) and the dicyclo to hexacyclo paraffin component (R2) was also calculated.

<Method for Measuring Various Physical Properties of Lubricating Oil Composition>

(10) HTHS Viscosity at 150° C. (High-Temperature High Shear Viscosity)

A lubricating oil composition to be measured was sheared at a shear rate of 10⁶/s at 150° C., and the viscosity after shearing was measured in accordance with ASTM D4741.

The “bottom oil” and the “slack wax” used in each of the Examples and Comparative Examples were produced as follows.

Production Example 1 (Production of Bottom Oil)

In a process of producing a typical feedstock oil, after naphtha and light kerosene were separated and removed by hydrocracking an oil containing a heavy fuel oil obtained by distilling a crude oil under reduced pressure by a vacuum distillation apparatus, the remaining bottom fraction was taken out. The bottom fraction was used as a “bottom oil” in the following production.

For the bottom oil, the content of the oil component=75 mass %, the content of the sulfur component=82 ppm by mass, the content of the nitrogen component=2 ppm by mass, the kinematic viscosity at 100° C.=4.1 mm²/s, and the viscosity index=134.

Production Example 2 (Production of Solvent Dewaxed Oil and Slack Wax)

The bottom oil obtained as described above was solvent dewaxed in a low temperature range of −35° C. to −30° C. using a mixed solvent of methyl ethyl ketone and toluene to separate a wax and obtain a “solvent dewaxed oil.” Then, the separated wax was referred to as a “slack wax.”

For the solvent dewaxed oil, the content of the oil component=100 mass %, the content of the sulfur component=70 ppm by mass, the content of the nitrogen component=2 ppm by mass, the kinematic viscosity at 100° C.=4.1 mm²/s, and the viscosity index=121.

In addition, for the slack wax, the content of the oil component=15 mass %, the content of the sulfur component=12 ppm by mass, the content of the nitrogen component=less than 1 ppm by mass, the kinematic viscosity at 100° C.=4.2 mm²/s, and the viscosity index=169.

Example 1 (Production of Mineral Base Oil (1))

The solvent dewaxed oil obtained in Preparation Example 2 was used as a feedstock oil (i).

The feedstock oil (i) was subjected to hydrogenation process under the conditions of a hydrogen partial pressure of 20 MPa, a reaction temperature of 280 to 320° C., and an LHSV of 1.0 hr⁻¹ by using a nickel-tungsten-based catalyst.

The hydrogenated product oil was distilled under reduced pressure to recover a fraction having a kinematic viscosity within a range of 4.2 to 4.4 mm²/s at 100° C., thereby obtaining a mineral base oil (1).

For the mineral base oil (1), the aromatic content (% C_(A))=0.0, the naphthene content (% C_(N))=26.5, the content of the sulfur component=less than 100 ppm by mass, and the weight average molecular weight=150 to 450.

Example 2 (Production of Mineral Base Oil (2))

A mixture of 75 parts by mass of the slack wax obtained in Preparation Example 2 and 25 parts by mass of the bottom oil obtained in Preparation Example 1 was used as a feedstock oil (ii). For the feedstock oil (ii), the content of the oil component=30 mass %, the content of the sulfur component=30 ppm by mass, the content of the nitrogen component=less than 1 ppm by mass, the kinematic viscosity at 100° C.=4.2 mm²/s, and the viscosity index=160.

The feedstock oil (ii) was subjected to hydroisomerization dewaxing under the conditions of a hydrogen partial pressure of 4 MPa, a reaction temperature of 335° C., and an LHSV of 1.0 hr⁻¹ by using a hydroisomerization dewaxing catalyst.

Subsequently, the hydroisomerization dewaxed product oil was subjected to hydrogenation process under the conditions of a hydrogen partial pressure of 20 MPa, a reaction temperature of 280 to 320° C., and an LHSV of 1.0 hr⁻¹ by using a nickel-tungsten-based catalyst.

The hydrogenated product oil was distilled under reduced pressure to recover a fraction having a kinematic viscosity within a range of 4.2 to 4.4 mm²/s at 100° C., thereby obtaining a mineral base oil (2).

For the mineral base oil (2), the aromatic content (% C_(A))=0.0, the naphthene content (% C_(N))=18.3, the content of the sulfur component=less than 100 ppm by mass, and the weight average molecular weight=150 to 450.

Example 3 (Production of Mineral Base Oil (3))

A mineral base oil (3) was obtained in the same manner as in the method in Example 2, except that in the production method in Example 2, the hydrogenated product oil was distilled under reduced pressure to recover a fraction having a kinematic viscosity within a range of 2.5 to 3M mm²/s at 100° C.

For the mineral base oil (3), the aromatic content (% C_(A))=0.1, the naphthene content (% C_(N))=20.2, the content of the sulfur component=less than 100 ppm by mass, and the weight average molecular weight=150 to 450.

Comparative Example 1 (Production of Mineral Base Oil (a))

The heavy fuel oil obtained from the vacuum distillation apparatus in the process of producing a typical fuel oil was solvent extracted under the condition of a solvent ratio of 1.0 to 2.0 by using a furfural solvent to obtain a raffinate.

Then, the raffinate was subjected to hydroisomerization dewaxing under the conditions of a hydrogen partial pressure of 4 MPa, a reaction temperature of 260 to 280° C., and an LHSV of 1.0 hr⁻¹ by using a hydroisomerization dewaxing catalyst.

Subsequently, the hydroisomerization dewaxed product oil was subjected to hydrogenation process under the conditions of a hydrogen partial pressure of 4 to 5 MPa, a reaction temperature of 280 to 320° C., and an LHSV of 1.0 hr⁻¹ by using a nickel-tungsten-based catalyst.

The hydrogenated product oil was distilled under reduced pressure to recover a fraction having a kinematic viscosity within a range of 4.0 to 4.5 mm²/s at 100° C., thereby obtaining a mineral base oil (a).

For the mineral base oil (a), the aromatic content (% C_(A))=2.8, the naphthene content (% C_(N))=27.3, the content of the sulfur component=1,000 ppm by mass, and the weight average molecular weight=150 to 450.

Comparative Example 2 (Production of Mineral Base Oil (b))

A mineral base oil (b) was obtained in the same manner as in the method in Comparative Example 1, except that the generated oil after the hydrogenation process in the production method of Comparative Example 1 was vacuum distillated, and that a fraction having a kinematic viscosity at 100° C. ranging from 2.0 to 3.0 mm²/s was collected.

For the mineral base oil (b), the aromatic content (% C_(A))=4.7, the naphthene content (% C_(N))=28.7, the content of the sulfur component=2,000 ppm by mass, and the weight average molecular weight=150 to 450.

Comparative Example 3 (Production of Mineral Base Oil (c))

A mixture of 20 parts by mass of the slack wax obtained in Production Example 2 and 80 parts by mass of the bottom oil obtained in Production Example 1 was used as a feedstock oil (iv). For the feedstock oil (iv), the content of the oil component=62.5 mass %, the content of the sulfur component=68 ppm by mass, the content of the nitrogen component=2 ppm by mass, the kinematic viscosity at 100° C.=4.1 mm²/s, and the viscosity index=141.

Then, a mineral base oil (c) was obtained in the same manner as in Example 2, except that in the production method in Example 2, the hydrogenated product oil was distilled under reduced pressure to recover a fraction having a kinematic viscosity at 100° C. within a range of 6.0 to 7.0 mm²/s by using the feedstock oil (iv) instead of the feedstock oil (ii) as a feedstock oil.

For the mineral base oil (c), the aromatic content (% C_(A))=0.0, the naphthene content (% C_(N))=21.4, the content of the sulfur component=less than 100 ppm by mass, and the weight average molecular weight=450 seconds.

Various properties of the mineral base oils produced in the Examples and Comparative Examples are shown in Table 1. In addition, the graph that represents the relationship between temperature and complex viscosity η* with respect to the mineral base oil (2) of Example 2, the mineral base oil (a) of Comparative Example 1, and the mineral base oil (b) of Comparative Example 2 is shown in FIG. 1.

TABLE 1 Example Comparative Example 1 2 3 1 2 3 Mineral Mineral Mineral Mineral Mineral Mineral base oil base oil base oil base oil base oil base oil Properties Unit (1) (2) (3) (a) (b) (c) Kinematic viscosity at 40° C. mm²/s 20.2 18.3 9.3 20.7 9.6 36.9 Kinematic viscosity at 100° C. mm²/s  4.3  4.2 2.6 4.2 2.6  6.5 Viscosity index — 123   132   113    103 97 130   CCS viscosity at −35° C. mPa · s 3700    2600    1000    4800 1200 9800    Temperature gradient Δ | η* | of complex viscosity between Pa · s/° C. 16.7 12.7 1.1 1633.3 323.3 63.4 two points of −10° C. and −25° C. Complex viscosity η* at −10° C. Pa · s   0.201   0.180  0.080 0.267 0.072   0.392 Complex viscosity η* at −25° C. Pa · s 250.3  190.1  17.30 24500.0 4850.0 951.1  Complex viscosity η* at −35° C. Pa · s 39000    6023    185.2  94240 80330 90320    Aromatic content (% C_(A)) —  0.0  0.0 0.1 2.8 4.7  0.0 Naphthene content (% C_(N)) — 26.5 18.3 20.2  27.3 28.7 21.4 Sulfur component Mass ppm 100>   100>   100>   1000 2000 100>   Type analysis of Acyclic paraffin component (R0) vol % 49.0 58.8 54.4  (*) (*) 41.8 paraffin component Monocyclo paraffin component (R1) vol % 16.5 15.4 14.4  (*) (*) 24.2 (content of each Bicyclo paraffin component vol % 12.4 10.7 10.4  (*) (*) 17.2 component) Tricyclo paraffin component vol %  6.4  4.8 7.2 (*) (*) 10.0 Tetracyclo paraffin component vol %  3.1  1.8 7.9 (*) (*)  1.7 Pentacyclo paraffin component vol %  7.4  4.7 5.6 (*) (*)  3.8 Hexacyclo paraffin component vol %  5.2  3.8 0.1 (*) (*)  1.3 Total vol % 100.0  100.0  100.0  (*) (*) 100.0  Dicyclo to hexacyclo paraffin vol % 34.5 25.8 31.2  (*) (*) 34.0 component (R2) R1/R2 (volume ratio) —  0.48  0.60  0.46 (*) (*)  0.71 (*): Since the value of “temperature gradient Δ | η * | of complex viscosity” was remarkably large, no measurement was made.

Examples 4 to 7 and Comparative Examples 4 to 8

Using any of the mineral base oils (1) to (3) and (a) to (c) produced in the Examples and the Comparative examples of the types shown in Table 2, lubricating oil compositions (i) to (iv) and (A) to (E) were respectively prepared by blending additives for lubricating oil, which had the types and blending amounts shown in Table 2.

Furthermore, the details of the additives for lubricating oil in Table 2 are as follows.

-   -   Viscosity index improver: an olefin-based copolymer having Mw of         500,000.     -   Metal-based detergent: overbased calcium salicylate, base number         (perchloric acid method)=225 mgKOH/g, and calcium atom content         7.8 mass %.     -   Anti-Wear Agent (1): secondary alkyl-type zinc dialkyl         dithiophosphate, zinc content=9.0 mass %, and phosphorus atom         content=8.2 mass %.     -   Antioxidant (1): an amine-based antioxidant.     -   Antioxidant (2): a phenol-based antioxidant.     -   Dispersant (1): polybutenyl succinic acid bisimide, Mn of a         polybutenyl group=2,000, base number (perchloric acid         method)=11.9 mgKOH/g, and nitrogen atom content=0.99 mass %.     -   Dispersant (2): polybutenyl succinic acid monoimide boride, Mn         of a polybutenyl group=1,000, base number (perchloric acid         method)=25 mgKOH/g, nitrogen atom content=1.23 mass %, and boron         atom content=1.3 mass %.     -   Rust inhibitor, Anti-foaming agent     -   Pour-point depressant: a polymethacrylate having Mw of 69,000.

Then, various properties of the prepared lubricating oil compositions (i) to (iv) and (A) to (E) were measured according to the above measuring method. Further, the panel coking test was carried out based on the following method to measure the deposit amount, and for the lubricating oil compositions (iv), (D) and (E) containing the pour-point depressant, a percentage increase P of the deposit amount was also calculated. These results are shown in Table 2.

[Panel Coking Test]

(1) Measurement of Deposit Amount

300 mL of the prepared lubricating oil composition was charged into a heating vessel and heated to 100° C. The lubricating oil composition heated to 100° C. was splashed onto an aluminum board heated to 300° C. and installed at an upper portion of the heating vessel by using continuously rotating blades at 1,000 rpm. This operation was continuously performed for 3 hours by repeating a “cycle consisting of a blade rotation for 15 seconds and a pause for 45 seconds”. After 3 hours, the mass of the deposit (deposit amount) adhered to the aluminum board was measured.

(2) Calculation of Percentage Increase P of Deposit Amount

Based on the deposit amount calculated in (1), the percentage increase P of the deposit amount (W) of the lubricating oil composition (iv) of Example 7 containing a pour-point depressant with respect to deposit amount (W₀) of the lubricating oil composition (I) of Example 4 containing no pour-point depressant was calculated based on the following calculation formula (f2).

P (unit: =(W−W ₀)/W ₀×100  Calculation Formula (f2):

In addition, likewise, the percentage increase P of the deposit amount (W) of the lubricating oil composition (D) of Comparative Example 7 containing a pour-point depressant with respect to the deposit amount (W₀) of the lubricating oil composition (A) of Comparative Example 4 containing no pour-point depressant, and the percentage increase P of the deposit amount (W) of the lubricating oil composition (E) of Example 8 containing a pour-point depressant with respect to deposit amount (W₀) of the lubricating oil composition of Comparative Example 5 containing no pour-point depressant were also calculated from the calculation formula (f2).

TABLE 2 Comparative Example Example 4 5 6 7 4 Lubricating oil composition (i) (ii) (iii) (iv) (A) Composition Mineral base oil Mineral Base Oil (1) obtained in mass % 86.30 — — 85.30 — Example 1 Mineral Base Oil (2) obtained in — 86.30 — — — Example 2 Mineral Base Oil (3) obtained in — — 86.30 — — Example 3 Mineral Base Oil (a) obtained in — — — — 86.30 Comparative Example 1 Mineral Base Oil (b) obtained in — — — — — Comparative Example 2 Mineral Base Oil (c) obtained in — — — — — Comparative Example 3 Additive for Viscosity index improver mass % 2.50 2.50 2.50 2.50 2.50 lubricating oil Metal-based detergent 3.00 3.00 3.00 3.00 3.00 Anti-Wear Agent (1) 1.40 1.40 1.40 1.40 1.40 Antioxidant (1) 0.50 0.50 0.50 0.50 0.50 Antioxidant (2) 0.50 0.50 0.50 0.50 0.50 Dispersant (1) 4.00 4.00 4.00 4.00 4.00 Dispersant (2) 1.00 1.00 1.00 1.00 1.00 Rust inhibitor, Anti-foaming agent 0.80 0.80 0.80 0.80 0.80 Pour-point depressant — — — 1.00 — Total mass % 100.00 100.00 100.00 100.00 100.00 Properties of used mineral base Kinematic viscosity at 100° C. mm²/s 4.3 4.2 2.6 4.3 4.2 oil Viscosity index — 123 132 113 123 103 Temperature gradient Δ | η* | of Pa · s/° C. 16.7 12.7 1.1 16.7 1633.3 complex viscosity between two points of −10° C. and −25° C. R1/R2 — 0.48 0.60 0.46 0.48 (*) % C_(A) — 0.0 0.0 0.1 0.0 2.8 Properties of lubricating oil Kinematic viscosity at 40° C. mm²/s 50.3 48.2 26.0 55.5 54.5 composition Kinematic viscosity at 100° C. mm²/s 9.1 9.0 6.0 10.0 9.3 Viscosity index — 164 169 189 170 152 Temperature gradient Δ | η* | of Pa · s/° C. 19.3 16.2 1.6 0.8 260.2 complex viscosity between two points of −10° C. and −25° C. Complex viscosity η* at −35° C. Pa · s 35520 4300 193.6 53.4 58300 CCS viscosity at −35° C. mPa · s 8400 7400 4700 8500 9900 HTHS viscosity at 150° C. mPa · s 3.4 3.4 2.5 3.7 3.4 Deposit amount mg 75 70 78 81 98 Percentage increase P of deposit % — — — 8.0 — amount Object to be — — — (i) — compared Comparative Example 5 6 7 8 Lubricating oil composition (B) (C) (D) (E) Composition Mineral base oil Mineral Base Oil (1) obtained in mass % — — — — Example 1 Mineral Base Oil (2) obtained in — — — — Example 2 Mineral Base Oil (3) obtained in — — — — Example 3 Mineral Base Oil (a) obtained in — — 85.30 — Comparative Example 1 Mineral Base Oil (b) obtained in 86.30 — — 85.30 Comparative Example 2 Mineral Base Oil (c) obtained in — 86.30 — — Comparative Example 3 Additive for Viscosity index improver mass % 2.50 2.50 2.50 2.50 lubricating oil Metal-based detergent 3.00 3.00 3.00 3.00 Anti-Wear Agent (1) 1.40 1.40 1.40 1.40 Antioxidant (1) 0.50 0.50 0.50 0.50 Antioxidant (2) 0.50 0.50 0.50 0.50 Dispersant (1) 4.00 4.00 4.00 4.00 Dispersant (2) 1.00 1.00 1.00 1.00 Rust inhibitor, Anti-foaming agent 0.80 0.80 0.80 0.80 Pour-point depressant — — 1.00 1.00 Total mass % 100.00 100.00 100.00 100.00 Properties of used mineral base Kinematic viscosity at 100° C. mm²/s 2.6 6.5 4.2 2.6 oil Viscosity index — 97 130 103 97 Temperature gradient Δ | η* | of Pa · s/° C. 323.3 63.4 1633.3 323.3 complex viscosity between two points of −10° C. and −25° C. R1/R2 — (*) 0.71 (*) (*) % C_(A) — 4.7 0.0 2.8 4.7 Properties of lubricating oil Kinematic viscosity at 40° C. mm²/s 28.2 53.2 60.3 34.4 composition Kinematic viscosity at 100° C. mm²/s 6.2 9.3 10.2 7.4 Viscosity index — 180 158 157 190 Temperature gradient Δ | η* | of Pa · s/° C. 66.1 65.0 2.5 0.5 complex viscosity between two points of −10° C. and −25° C. Complex viscosity η* at −35° C. Pa · s 56535 80823 85.0 123.0 CCS viscosity at −35° C. mPa · s 6000 14800 9800 6000 HTHS viscosity at 150° C. mPa · s 2.5 3.6 3.7 2.9 Deposit amount mg 108 95 118 125 Percentage increase P of deposit % — — 20.4 15.7 amount Object to be — — (A) (B) compared

The lubricating oil compositions (i) to (iv) of Examples 4 to 7 using the mineral base oils (1) to (3) obtained in Examples 1 to 3 are good in low-temperature viscosity characteristics, and the deposit amount by the panel coking test was also low, resulting in excellent high-temperature detergency of a piston.

Meanwhile, the lubricating oil compositions (A), (C) and (D) of Comparative Examples 4, 6 and 7 using the mineral base oils (a) and (c) obtained in Comparative Examples 1 and 3 resulted in deterioration in low-temperature viscosity characteristics. Furthermore, the lubricating oil compositions (A) to (E) of Comparative Examples 4 to 8 using the mineral base oils (a) to (c) obtained in Comparative Examples 1 to 3 had a problem with high-temperature detergency of a piston due to a high deposit amount.

Further, when the values of the percentage increase P of the deposit amount between the lubricating oil composition (iv) of Example 7 and the lubricating oil compositions (D) and (E) of Comparative Examples 7 and 8 are compared, it can be seen that by using the mineral base oil of the present invention, the effect of suppressing deposit generation is high even though the mineral base oil contains a pour-point depressant.

REFERENCE SIGNS LIST

-   -   1: Sliding mechanism     -   2: Block     -   2 a: Piston travel path     -   2 b: Crank shaft housing     -   4: Piston     -   6, 8: Piston ring     -   9: Connecting rod     -   10: Crank shaft     -   12: Liner     -   20: Lubricating oil composition 

1. A mineral base oil satisfying the following requirements (I) to (V): Requirement (I): a kinematic viscosity at 100° C. is 2 mm²/s or more and less than 7 mm²/s; Requirement (II): a viscosity index is 100 or more; Requirement (III): a temperature gradient Δ|η*| of complex viscosity between two points of −10° C. and −25° C. is 60 Pa·s/° C. or less as measured with a rotary rheometer under conditions at an angular velocity of 6.3 rad/s and a strain amount of 0.1 to 100%; Requirement (IV): a content ratio [R1/R2] of a monocyclo paraffin component (R1) to a dicyclo to hexacyclo paraffin component (R2) is 0.70 or less by a volume ratio as measured in accordance with ASTM D2786; and Requirement (V): a % C_(A) is less than 1.0.
 2. The mineral base oil according to claim 1, wherein a total content of a monocyclo paraffin component (R1) and a dicyclo to hexacyclo paraffin component (R2) is 20 vol % or more based on 100 vol % of a total amount of paraffin components contained in the mineral base oil as measured in accordance with ASTM D2786.
 3. The mineral base oil according to claim 1, wherein a content of the dicyclo to hexacyclo paraffin component (R2) is from 15 to 70 vol % based on 100 vol % of the total amount of paraffin components contained in the mineral base oil as measured in accordance with ASTM D2786.
 4. The mineral base oil according to claim 1, wherein a complex viscosity η* at −35° C. is 60,000 Pa·s/° C. or less as measured with a rotary rheometer under conditions at an angular velocity of 6.3 rad/s and a strain amount of 0.1%.
 5. The mineral base oil according to claim 1, which has a naphthene content (% C_(N)) is 15 to
 30. 6. The mineral base oil according to claim 1, which has a viscosity index of 145 or less.
 7. The mineral base oil according to claim 1, wherein a content of a sulfur component is less than 100 ppm by mass based on the total amount of the mineral base oil.
 8. The mineral base oil according to claim 1, which has a weight average molecular weight of 450 or less.
 9. The mineral base oil according to claim 1, which is obtained by purifying a feedstock oil containing a wax.
 10. The mineral base oil according to claim 1, which is obtained by purifying a feedstock oil containing a wax and a bottom oil.
 11. The mineral base oil according to claim 10, wherein a content ratio [wax/bottom oil] of the wax and the bottom oil is from 30/70 to 95/5 by a mass ratio.
 12. A lubricating oil composition, comprising the mineral base oil according to claim
 1. 13. The lubricating oil composition according to claim 12, wherein a content of an additive for lubricating oil including a polymer component is 10 mass % or less based on a total amount of the lubricating oil composition.
 14. The lubricating oil composition according to claim 12, which has a kinematic viscosity at 100° C. of 4 mm²/s or more and less than 15 mm²/s, and a viscosity index of 140 or more.
 15. The lubricating oil composition according to claim 1, which has a kinematic viscosity at 100° C. of less than 12.5 mm²/s, and a high-temperature high shear viscosity (HTHS viscosity) at 150° C. of less than 3.5 mPa·s.
 16. An internal combustion engine comprising a sliding mechanism equipped with a piston ring and a liner; and the lubricating oil composition according to claim
 12. 17. A method of lubricating an internal combustion engine having a sliding mechanism equipped with a piston ring and a liner, the method comprising lubricating the piston ring and the liner with the lubricating oil composition according to claim
 12. 